To rotate a motor at a high speed with a large torque is one of the most important factors in terms of an improvement in basic performance of the motor, and has been studied and developed for many years. For example, an electronic timepiece as one of products using motors has been advanced to have many functions in recent years. Timepieces having various functions such as stopwatch function, an alarm function, and a dual time function, in addition to normal time display have been developed and commercially available. These multifunctional timepieces always perform the fast forwarding operation of pointing hands when the system is initialized in an initial state, e.g., in loading the battery, or when the mode is shifted or the hand position is zeroed in normal use. For this reason, high-speed rotation of the motor is very important in terms of an improvement in operability, and the like.
A large-torque motor is indispensable when a flat weight is attached to a motor to use a timepiece as a vibration alarm for informing the time using vibrations generated upon rotation of the motor, or when a disk is attached instead of a pointing hand to a timepiece motor to perform display other than display of the time.
When the motor is rotated at a high speed, even if output of a driving pulse is stopped, the motor may not immediately stop due to the inertia of the motor or parts (e.g., a reduction ring train and a pointing hand in the timepiece) connected to the motor. Normally, to obtain a predetermined number of rotation, a corresponding number of pulses are output. In this case, however, the number of output pulses is not equal to the number of rotation of the motor.
In, e.g., a timepiece pointing hand, this is a fatal defect and leads to a time display error and the like. In addition, the magnetic pole generated at the stator by a driving pulse output in driving the motor next may shift from the pole position of the rotor depending on the pole position of the rotor upon stopping the motor, and the motor may not normally rotate. Therefore, in a system for driving the motor at a high speed, and accurately controlling the rotation speed of the motor to a desired value, it is important that the motor must be stopped at a predetermined position.
A conventional motor driving apparatus will be described below by exemplifying the stepping motor of an electronic timepiece.
FIG. 1 is a view of the arrangement of a motor driving apparatus constituted by a conventional bipolar stepping motor, and FIGS. 2 to 7 are plan views, respectively, showing the positional relationship between the magnetic poles of a stator and a rotor. As a means for converting an electrical signal into a mechanical rotating motion, the bipolar stepping motor is constituted by a driving coil 101, a flat stator 102, and a rotor 103, as shown in FIG. 1. The flat stator 102 has a step 102a, as shown in FIG. 2.
Motor drivers 104a and 104b are arranged to cause a current to flow through the driving coil 101 by changing the potential across the two terminals of the driving coil 101, thereby exciting the flat stator 102. In the bipolar motor shown in FIG. 1, when no current flows through the driving coil 101, the pole position of the rotor 103 with respect to the flat stator 102 is at a statically stable point 110 shown in FIG. 2; when a current flows through the driving coil 101 to excite the flat stator 102, it is at an electromagnetically stable point 111 shown in FIG. 3.
Normally, in the electronic timepiece, a driving pulse signal for changing the potential across the two terminals of the driving coil 101 is output from the motor driver 104a or 104b for 4 to 5 mS to cause a pulse current to flow through the driving coil 101, thereby rotating the rotor 103. When the rotor 103 rotates during the supply of the current to the driving coil 101, and comes to nearly a pole position shown in FIG. 4 with respect to the flat stator 102, the current flowing through the driving coil 101 stops. The rotor 103 rotates to a position in FIG. 5 due to the inertia, is subjected to damped vibration about the statically stable point 110, and finally stops.
After the rotor 103 becomes still, a driving pulse signal is output from, e.g., the motor driver 104a to cause a current to flow through the driving coil 101, thereby exciting the flat stator 102, as shown in FIG. 6. In this case, the rotor 103 rotates through 180.degree. in a rotation direction A shown in FIG. 6. Further, after the rotor 103 becomes still, a driving pulse signal is output from the motor driver 104b on a side opposite to the side from which the previous driving pulse signal is output. Then, the rotor 103 rotates another 180.degree. in the direction A in FIG. 6. If the rotor 103 is rotated by causing a current to flow through the driving coil 101 after the rotor 103 becomes still, it reliably rotates in the direction A shown in FIG. 6.
When the stepping motor is to be rotated at a high speed, the rotor 103 must be rotated at a high speed, as matter of course. At this time, the output interval between driving pulse signals output from the motor drivers 104a and 104b must be shortened.
If the output interval between driving pulse signals is shortened in order to rotate the rotor 103 at a higher speed, the next driving pulse signal must be output before damped vibration of the rotor 103 immediately after its rotation stop.
If the next driving pulse signal is output when the rotor 103 is at a position in FIG. 7 during damped vibration, i.e., the rotor 103 and the electromagnetically stable point 111 have a positional relationship shown in FIG. 7, the rotor 103 undesirably rotates in a direction opposite to the direction A shown in FIG. 6, i.e., opposite to the normal direction. Therefore, to stably rotate the rotor 103, the output interval between driving pulse signals must be set to a time or more required to stabilize damped vibration of the rotor 103 upon rotation within a range not to exceed the electromagnetically stable point 111.
Even the minimum total time of the pulse width of the driving pulse signal and the damped-vibration stabilization time of, i.e., the minimum output period of the driving pulse signal is around 10 mS. This indicates that the output frequency of the driving pulse signal is limited to about 100 Hz in the current driving scheme.
This problem, however, has been solved by the scheme disclosed in Japanese Patent Application No. 6-304440 filed by the present applicant.
FIG. 8 is a circuit diagram showing an example of a driving circuit in a conventional motor driving apparatus, and FIG. 9 is a waveform chart showing the operation of the driving circuit in FIG. 8.
In FIG. 8, reference numeral 25' denotes a driving circuit, which is constituted by motor drivers 1a and 1b. Reference numeral 2 denotes a driving coil; and 41', a counter electromotive voltage detection circuit, which has a bias means 3 and a voltage detection circuit 5. The bias means 3 is constituted by switch means 3a and 3b, and bias resistors 3c and 3d having the same resistance value. Reference numeral 4 denotes a flat stator. The voltage detection circuit 5 is constituted by an inverter 5a, a feedback resistor 5b, and an input resistor 5c. Reference numeral 6 denotes an inverter; 103, a rotor; and 42', a motor constituted by the driving coil 2, the flat stator 4, and the rotor 103.
When a signal OE1 is at "H" level, the motor drivers 1a and 1b buffer-output input signals O1in and O2in, respectively; when the signal OE1 is at "L" level, their outputs are set in a high-impedance state. The switch means 3a and 3b are switches which are turned off when a signal SE output from the inverter 6 is at "L" level, and turned on when the signal SE is at "H" level.
The operation of the circuit in FIG. 8 will be explained with reference to the waveform chart of FIG. 9.
During a period (a) in FIG. 9, since the signal OE1 is at "H" level, and an "H"-level driving pulse signal is output from the motor driver 1a, a current flows through the driving coil 2 to rotate the rotor 103. During this period, both the switch means 3a and 3b are in the OFF state because the signal SE is at "L" level. During a period (b) in FIG. 9, since the signal OE1 is at "L" level, outputs from the motor drivers 1a and 1b are in the high-impedance state, and the switch means 3a and 3b are turned on. The voltage at a terminal X as one terminal of the driving coil 2 is divided into a bias voltage Vb as a voltage 1/2 the power supply voltage.
A voltage waveform appearing at a terminal Y, as the other terminal of the driving coil 2, during the period (b) in FIG. 9 will be explained.
When outputs from the motor drivers 1a and 1b are in the high-impedance state, the switch means 3a and 3b are ON, and the voltage at the terminal X is set at the level of the bias voltage Vb by the bias resistors 3c and 3d, the voltage value at the terminal Y becomes the bias voltage Vb, similar to the terminal X, as far as the rotor 103 does not rotate and the motor drivers 1a and 1b have no influence. However, immediately after a driving pulse signal is output during the period (a) in FIG. 9, a current flowing through the driving coil 2 is stopped to generate an induced voltage as Vr in FIG. 9. When a driving pulse signal is output to rotate the rotor 103, a counter electromotive voltage Vg is generated upon rotation of the rotor 103, as shown in FIG. 9. The synthesized waveform of these generated voltages appears at the terminal Y. The voltage waveform appearing at the terminal Y is amplified by the voltage detection circuit 5 to have a waveform indicated by Aout in FIG. 9.
In the waveform Aout during the period (b) in FIG. 9, the induced voltage generated from the driving coil 2 is dominant immediately after the driving pulse signal is output. With the lapse of time, the influence of the induced voltage decreases, while the counter electromotive voltage from the rotor 103 becomes dominant.
In FIG. 9, a timing (time P) when the waveform Aout crosses the bias voltage Vb from the positive direction to the negative direction becomes almost equal to a timing when the rotor 103 passes the described-above electromagnetically stable point. If a driving pulse signal is output at this timing from the motor driver 1b on a side opposite to the side from which the previous driving pulse signal is output, the rotor 103 continuously rotates in the forward direction without reversely rotating because its pole position with respect to the flat stator 4 has already passed the electromagnetically stable point.
FIG. 10 shows another prior art in which a counter electromotive voltage generated from a motor is detected by a detection coil wound coaxially with a driving coil. The above-described stepping motor in FIG. 1 further comprises a voltage detection means constituted by a detection coil 105 wound coaxially with the driving coil 101, a differential amplifier 106a for detecting a counter electromotive voltage generated at the detection coil 105 upon rotation of the rotor 103, and a comparator 108 for comparing an output signal from the differential amplifier 106a with a reference voltage Vb, and outputting a signal Aout as the comparison result.
In FIG. 10, reference numeral 25 denotes a driving circuit constituted by motor drivers 104a and 104b; 41, a counter electromotive voltage detection circuit constituted by the detection coil 105 wound on a stator 102, the differential amplifier 106a, and the comparator 108; and 42, a motor constituted by the driving coil 101, the stator 102, and the rotor 103.
In this prior art, the pole position of the rotor 103 with respect to the flat stator 102 during rotation of the rotor 103 is detected by detecting the counter electromotive voltage generated upon rotation of the rotor 103 by using the voltage detection means through the detection coil 105, and the output timing of the driving pulse signal is controlled on the basis of an output from the comparator 108.
The motor having the arrangement of this prior art can be driven similarly to the scheme of detecting the counter electromotive voltage from the motor by using the above-described driving coil 2.in FIG. 8. FIG. 11 shows waveforms in this prior art. In this arrangement, since the DC component of a current flowing through the driving coil 101 is removed in outputting the driving pulse, a voltage waveform appearing on an output from the differential amplifier is the synthesized waveform of Vg and Vr.
In this prior art, the counter electromotive voltage is detected by the detection coil 105 wound coaxially with the driving coil 101. This scheme has already been filed by the present applicant as Japanese Unexamined Patent Publication No. 6-235777.
As described above, according to the driving scheme shown in FIGS. 8 and 9, the output interval between driving pulse signals can be minimized. As a result, the motor can be rotated at a speed about 3 times higher than that of a normal step driving scheme.
In this conventional synchronization driving scheme, the driving pulse conditions at the start of the motor are greatly different from those when the rotation speed is stabilized a predetermined time after the start. Therefore, several kinds of driving pulse signals to be supplied to the driving circuit are prepared in advance. A driving pulse signal having a large width is supplied to the driving circuit at the start of the motor, and the pulse width of the driving pulse signal to be supplied is decreased along with an increase in rotation speed.
However, in a system wherein a motor is attached with a heavy load, and particularly with an unbalanced load like a flat weight for a vibration motor, the driving pulse conditions greatly change depending on the posture of the motor. That is, an energy required at the start is greatly different between a case wherein the rotating shaft of the motor is perpendicular to the gravity, and the flat weight is located at such a position as to start rotation against the gravity, and a case wherein the flat weight is located at such a position as to start rotation in accordance with the gravity. As a result, the width condition of the driving pulse output from the driving circuit changes.
When the motor is driven by the conventional driving scheme, it cannot smoothly start because the pulse width at the start is fixed under predetermined conditions. That is, when the flat weight starts rotation against the gravity, the pulse width may not be long enough to start the motor, and the motor may fail to rotate. When the flat weight starts rotation in accordance with the gravity, the pulse width is excessive, resulting in an increase in power consumption.
In the scheme of driving the motor in synchronism with the phase angle of the rotor, which is a feature of the conventional driving scheme, the next driving pulse signal must be output at the timing when the motor rotates to reach an opposite phase. If a pulse having an excessive width is output, the same driving pulse is kept output even after the motor reaches the opposite phase. As a result, the motor is braked to greatly lower the rotation efficiency.
In the scheme of gradually decreasing the pulse width of a driving pulse signal within a predetermined time after the start of the motor, when the load of the motor is heavy, or when the driving voltage is low, the pulse width is decreased before the rotation speed of the motor sufficiently increases. As a result, not only the acceleration performance of the motor may be lowered, but also the energy necessary for rotation may not be obtained to stop the motor in some cases.
When the pulse width of the driving pulse signal is not sufficiently decreased with respect to the rotation speed, i.e., when a driving pulse signal having an excessive pulse width is output, the signal Aout output from the voltage detection circuit 5 in FIG. 8 becomes the one shown in FIG. 12. That is, the counter electromotive voltage generated from the motor shifts to the negative side with respect to the potential Vb before the influence of the induced voltage generated after outputting a driving pulse signal disappears. For this reason, no next driving pulse signal which is supposed to be output at the time Q is output. As a result, the motor stops, or even if it does not stop, the rotation speed does not increase.